DISCLAIMER: I haven't tried this myself, so I'm not entirely sure it will work, but...
You might be able to get it to work by editing your channel list to only show the OTA channel (e.g., show 7-1, but not cable channel 7, analog OTA channel 7, or a nonexistent DirecTV channel 7). Then, when you enter "7", it might tune 7-1.
I have a Zenith HD-SAT520, and I seem to recall some 520 users reporting success with this strategy.
However, you would probably also need to hack the IR codeset (I think The Robman has done some work in this regard) in the Replay to get it to tune your STB, and even then the channel will have to correspond to an existing lineup available to the Replay if you want to do anything other than manual recordings.